This invention relates to electronic digital communication systems, for example radiotelephone systems.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS and WCDMA standards. This application focusses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
WCDMA is based on direct-sequence spread-spectrum techniques, with pseudo-noise scrambling codes and orthogonal channelization codes separating base stations and physical channels (terminals or users), respectively, in the downlink (base-to-terminal) direction. Since all users share the same radio resource in CDMA systems, it is important that each physical channel does not use more power than necessary. This is achieved by a transmit power control (TPC) mechanism, in which, among other things, base stations send TPC commands to users in the downlink (DL) direction and the users implement the commands in the uplink (UL) direction. The TPC commands cause the users to increase or decrease their transmitted power levels by increments, thereby maintaining target signal-to-interference ratios (SIRs) for the dedicated physical channels (DPCHs) between the base stations and the users. WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology. Scrambling and channelization codes and transmit power control are well known in the art.
FIG. 1 depicts a mobile cellular radio telecommunication system 10, which may be, for example, a WCDMA communication system. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. More generally, each RNC directs mobile station (MS), or user equipment (UE), calls via the appropriate base station(s) (BSs), which communicate with each UE through DL, or forward, and UL (i.e., mobile-to-base, or reverse) channels. RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26. Each BS, which is called a Node B in 3GPP parlance, serves a geographical area that can be divided into one or more cell(s). BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26. The BSs are coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. Both RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the Internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
In such a communication system, each BS transmits predetermined pilot symbols on the UE's DPCH. The BS also transmits pilot symbols on a common pilot channel (CPICH), and a UE typically uses the CPICH pilot symbols in estimating the impulse response of the radio channel to the BS. It will be recognized that the UE uses the CPICH pilots for channel estimation, rather than the DPCH pilots, due to the CPICH's typically higher signal-to-noise ratio (SNR), but the UE still uses the DPCH pilots, mainly for SIR estimation, i.e., for DL power control.
High-speed downlink packet access (HSDPA) is an evolution of WCDMA communication systems that provides higher bit rates, e.g., up to more than 10 megabits per second (Mb/s), by using higher-order modulation, e.g., 16-ary quadrature amplitude modulation (16-QAM), multiple spreading codes, e.g., up to fifteen codes with spreading factors of 16, and DL-channel feedback information. HSDPA is described in, for example, the Release 5 version of the system specifications promulgated by the 3GPP. The DL-channel feedback information is information sent by a UE to a BS through the UL channel regarding the DL channel's quality. The BS uses that information to optimize DL modulation and coding for throughput.
HSDPA also introduces time division multiplex (TDM) in WCDMA by transmitting, in time chunks using excess channel transmit power that a BS may have, to one or a few UEs (typically the UE or UEs that have the best DL channel(s)). The excess channel transmit power ECe is just the difference between the total available channel transmit power ECmax and the transmit power in current use for other channels ECother channels. The other channels include all common channels and DPCHs.
HSDPA also employs a hybrid automatic repeat request (ARQ) scheme on the physical layer in order to reduce the round-trip delay of erroneous received packets. The hybrid ARQ scheme involves transmission by the UE of acknowledgment (ACK) and non-acknowledgment (NACK) messages to the BS providing HSDPA service. This BS may be called the “serving” BS or cell. The HS-channels in the DL are transmitted only from the HSDPA serving cell, and HSDPA UL control signaling (including ACK/NACK and DL-channel quality reports) is detected by only the HSDPA serving cell.
As user terminals move with respect to the base stations, and possibly vice versa, on-going connections are maintained through a process of hand-off, or handover. For example in a cellular telephone system, as a user moves from one cell to another, the user's connection is handed over from one base station to another. Early cellular systems used hard handovers (HHOs), in which a first cell's base station (covering the cell that the user was leaving) would stop communicating with the user just as the second base station (covering the cell that the user was entering) started communication. Modern cellular systems typically use diversity, or soft, handovers (SHOs), in which a user is connected simultaneously to two or more base stations. In FIG. 1, MSs 28, 30 are shown communicating with plural base stations in diversity handover situations. MS 28 communicates with BSs 16, 18, 20, and MS 30 communicates with BSs 20, 22. A control communication link between the RNCs 12, 14 permits diversity communications to/from the MS 30 via the BSs 20, 22.
HSDPA can be used in mobility situations, e.g., where a UE and the BS(s) move with respect to one another, but soft handover is not specified for HSDPA channels. HSDPA channels support only hard handover. Therefore, there can be many situations in which a UE uses SHO for its DPCH(s) at the same time that it uses HHO for its HSDPA channel(s). The HS-channels in the DL are transmitted only from the HSDPA serving cell and HSDPA UL control signaling (including ACK/NACK and channel quality reports) is detected by only the HSDPA serving cell.
FIG. 2A depicts a typical one of the situations in which a UE 202 is in a SHO situation for non-HSDPA channel(s) and is using services transported through HSDPA channels. The UE 202 has simultaneous connections with BS 204 and BS 206 via dedicated physical data channels (DPDCHs) and dedicated physical control channels (DPCCHS) in both the UL and DL. In short, the UE 202 is in SHO with respect to these non-HSDPA channels. The BSs 204, 206 are controlled by an RNC 208. On the DPDCHs, higher-layer network signaling and maybe also speech or video services are transmitted. The DPCCH carries physical layer control signaling (e.g., pilot symbols/sequences, TPC commands, etc.). A CPICH is also provided in the DLs from the base stations.
FIG. 2B is similar to FIG. 2A in that it depicts the UE 202 having multiple simultaneous connections with BS 204 and a BS 206 (not shown) via DPDCHs and DPCCHs in the UL and the DL. RNC 208 (not shown) controls BS 204 and BS 206. In support of setting up a packet data session through HSDPA, the UE 202 measures on a regular basis (typically five times per second) the average SNR EC/I0 on the CPICHs from all of the base stations, or cells, in its “Active Set”, which are the base station(s) connected to the UE. This SNR is usually called the CPICH RSCP/RSSI and is given by:EC/I0=RSCP/RSSI  (1)where RSCP is received signal code power (in this case, on the CPICH) and RSSI is received signal strength indicator. The best cell, i.e., the base station whose CPICH is received by the UE with the highest EC/I0, will be the HSDPA serving cell. In FIG. 2B, the SNR of BS 206 is indicated as greater than the SNR of BS 204.
FIG. 2C is also similar to FIGS. 2A and 2B. In FIG. 2C, an HSDPA session has been set up between BS 206 and UE 202, and the figure indicates by dashed lines the HSDPA channels, i.e., a DL high-speed packet data shared channel (HS-PDSCH) carrying HS data packets, a DL high-speed shared control channel (HS-SCCH) carrying DL control information for the HS data packets, and an UL high-speed dedicated physical control channel (HS-DPCCH) carrying ACK/NACK reports and DL channel quality information. The DL HSDPA channels are transmitted only by the HS serving cell (BS 206 in FIG. 2C), and the UL HSDPA channel is received only by the HS serving cell. HS data packets are also exchanged by the serving BS 206 and the RNC 208.
Problems can arise because, as currently defined by the HSDPA specifications, the HS serving cell is selected as the cell having the highest CPICH EC/I0 but that parameter does not always correspond to the actual SNR for HSDPA detection. Hence, the best HS serving cell according to CPICH EC/I0 is not necessarily the best HS serving cell with respect to HSDPA throughput. This can be understood from the following example.
Assume a two-cell scenario, such as that depicted in FIGS. 2A, 2B, 2C, in which the number of receiver taps, or fingers, devoted to radio channel paths from BS A and BS B are LA, LB, respectively. Also assume that the average total transmitted BS powers are ECAtot, ECBtot, respectively, and the maximum available BS power is ECmax and is the same for both BSs.
Then, the CPICH EC/I0 for BS A is given by the following expression:
                                                                                          (                                                            E                      c                                                              I                      o                                                        )                                A                                                    =                                            ∑                              j                =                1                                            L                A                                      ⁢                          E              c                              A                ,                j                                                                                        ∑                                  j                  =                  1                                                  L                  A                                            ⁢                              E                c                                  j                  ,                  Atot                                                      +                                          ∑                                  j                  =                  1                                                  L                  B                                            ⁢                              E                c                                  j                  ,                  Btot                                                      +                          σ              2                                                          (        2        )            where ECA,j is the CPICH power for finger j, and σ2 is the noise power. Hence, EC/I0 is the sum of CPICH power over all fingers, or paths, divided by the total received signal and noise power. It will be understood that channel estimates may be implicitly included in Eq. 2 and other equations described below.
At the same time, it is known in the art that the HSDPA performance (assuming a RAKE receiver) is proportional to the SIR of the HSDPA channel, e.g., from BS A, that is given by the following expression:
                              SIR          HSDPA          A                ≈                              ∑                          I              =              1                                      L              A                                ⁢                                                    E                c                                  I                  ,                  max                                            -                              E                c                                  I                  ,                  Atot                                                                                                      ∑                                      j                    ≠                    1                                                  ⁢                                  E                  c                                      j                    ,                    Atot                                                              +                                                ∑                                      j                    =                    1                                                        L                    B                                                  ⁢                                  E                  c                                      j                    ,                    Btot                                                              +                              σ                2                                                                        (        3        )            which can be seen as the excess BS power divided by the non-orthogonal noise.
Examples can easily be found where BS A will be the HS serving cell (best cell according to Eq. 2) at the same time that BS B (or some other cell) will have the highest potential HSDPA SIR (according to Eq. 3). One such example is to let LB=1, LA=2 (with equal path strength), and ECBtot<ECAtot, which is to say that BS B has lower average load than BS A, and to let CPICH ECA=ECB+Δ, which is to say that a slightly stronger CPICH is received from BS A compared to BS B. In such a case, the UE will use BS A as the serving HS cell but a better throughput (and system utilization) could be achieved using BS B instead. It will be noted that the channel for BS B is only one tap, making the intra-cell interference orthogonal.